Biofouling which is formed from the microbes growing on surface of a membrane separation device, especially the biofouling on surface of a reverse osmosis membrane or nanofiltration membrane separation device will add significant negative affect on the osmotic pressure of the membrane separation device such as reverse osmosis membrane (RO membrane). Biofouling also reduces the flow rate and quality of the water produced by the membrane separation device, and increases the operation pressure and pressure drop of the system, and affects the operation of the membrane separation device significantly. During the operation of the membrane separation device, the chemicals which are applied to wash the membrane separation device are usually needed. However, such operation may reduce the service life of the membrane.
Biofouling is a sticky substance that adheres to surface of the objects, which is formed from that the microbes and viscous liquid produced by thereof are mixed with the other organic and inorganic substance. The membrane separation device is usually used to treat water. Organic impurities, inorganic impurities and microbes exist in the water system, and therefore it is easy for biofouling to form on the surface of a membrane separation device. In order to prevent the biofouling adversely affects the membrane separation device, the application of biocides is needed to inhibit the growth of the microbes so as to prevent the generation of biofouling.
Up to now, several non-oxidizing biocides such as DBNPA and isothiazolone have been applied to inhibit the biofouling in RO membrane device. These non-oxidizing biocides can control the growth of the biofouling to a certain level without destroying the membrane material itself of the membrane separation device. However these non-oxidizing biocides are too expensive and normally used in intermittent dosage, and sometimes can not guarantee the sufficient biocidal effect.
Due to the structure of the formed biofouling or biofilm is very dense, and it is typical the composite of the bacteria and extracellular matrix. Therefore, biofouling provides some protection to the organism therein. Usually the dosage of biocides used to kill the microbes in biofouling is 10 to 1000 times higher than that used to kill the same microbes which are obtained by normal cultivation. Additionally, if the growth of biofouling can not be inhibited thoroughly or the biofouling grown can not be removed clearly, the biofouling of microbes which is only killed to a certain level will further provide nutrient for the growing of microbes and promote the growth of microbes reversely. Based on the above, the cost of using non-oxidizing biocides is high, and the killing effect is insufficient, meanwhile it is hard to ensure an effective control on biofouling. Thus, there is a need for developing more effective and lower cost biocides to control the biofouling in the membrane separation device.
Chlorine is a very cheap biocide, and is the most frequently used as biocide, especially for water treatment process. A large amount of chlorine in water may exist in the form of free chlorine. Such free chlorine will oxidize the surface of membrane and reduce the separation ability of membrane, for example, destroy amide bonds of polyamide of RO membrane. Thus, in the water treatment system that typically consists of RO membrane, the chlorine is mainly applied for pretreatment. A dechlorination process within the water environment prior to RO membrane filtration must be performed using reducing agent such as sodium bisulfite. It is impossible to apply the effective chlorine-type biocide to the membrane separation device directly.
Recently, researchers have attempted to use different modified methods to control the biofouling of reverse osmosis membrane separation device or nanofiltration membrane separation device. For example, Louie et al. provided a membrane surface modification technique (J. S. Louie, I. Pinnau, I. Ciobanu, K. P. Ishida, A. Ng, M. Reinhard, Effects of polyether-polyamide block copolymer coating on performance and fouling of reverse osmosis membranes, J. Membr. Sci. 280 (2006) 762-770). The method of applying enzyme was developed by Richards et al. (M. Richards, T. E. Cloete, Nanoenzymes for biofilm removal, In: in: T. E. Cloete, M. Dekwaadsteniet, M. Botes, J. M. Lopez-Romero (Eds.), Nanotechnology in Water Treatment Applications, Caister Academic press, Norfolk, U K, 2010, pp. 89-102). Hilal et al. used filtration process to reduce the concentration of microbes (N. Hilal, H. Al-Zoubi, N. A. Darwish, A. W. Mohamma, M. Abu Arabi, A comprehensive review of nanofiltration membranes: treatment, pretreatment, modeling, and atomic force microscopy, Desalination 170 (2004) 281-308.). Wolf et al. disclosed that remove the nutrient in the feed stream of RO system through filtration process (P. H. Wolf, S. Siverns, S. Monti, UF membranes for RO desalination pretreatment, Desalination 182 (2005) 293-300.). Furthermore, the technique of using UV light (T. Harif, H. Elifantz, E. Margalit, M. Herzberg, T. Lichi, D. Minz, The effect of UV pre-treatment on biofouling of BWRO membranes: a field study, Desalin. Water Treat. 31 (2011) 151-163.), the technique of using electricity (M. I. Kerwick, S. M. Reddy, A. H. L. Chamberlain, D. M. Holt, Electrochemical disinfection, an environmentally acceptable method of drinking water disinfection Electrochim. Acta 50 (2005) 5270-5277) and the technique of using ultrasonic wave (R. A. Al-Juboori, T. Yusaf, V. Aravinthan, Investigating the efficiency of thermosonication for controlling biofouling in batch membrane systems, Desalination 286 (2012) 349-357.) were disclosed respectively. These techniques are still associated with some shortcomings, for example, high cost, insufficient activity on inhibiting biofouling, low stability and applicability. Thus, there is still a need for developing an effective biological inhibitor and a method for controlling biofouling.
The method of controlling biofouling with chemicals is still the most concerned researching directions. The research which used chloramine as biocide was attractive. However when transition metal ions such as Fe ions exist, chloramine still damages RO membrane. Moreover, chloramine is highly influenced by pH during practical operation. When pH reduces to less than 6, chlorine may be produced and it may lead to a potential safety hazard.
Sulfamic acid is a common cleaning agent and also considered as an environment friendly biocide. Sulfamic acid and sulfamate can be used to stabilize halogens and form sulfamic acid halogen products that have a function of preventing bacteria from adhesion to metal or plastic surfaces (U.S. Pat. No. 6,380,182B1 (2002), Thomas E. McNeel, Marilyn S. Whittemore, Stephen D. Bryant, Graciela H. Vunk, Methods and Compositions Controlling Biofouling Using Sulfamic Acids.).
It is noticed that chlorine stabilized by using sulfamic acid can effectively reduce the level of free chlorine in environment without sacrificing total active chlorine. Therefore, the risk of membrane damaged by free chlorine can be minimized as much as possible and an opportunity that utilizes cheaper chlorine as biocides to treat membrane separation device may be provided. Additionally, sulfamic acid-based substances themselves are low cost products and suitable for application in large amounts. Kurita Company has used sulfamic acid to stabilize chlorine and used it as an inhibitor for biofouling of RO membrane (JP 2006-263510). However, the chlorine stabilized by sulfamic acid fails to provide sufficient activity on controlling the growth of biofouling, and there is a limitation on the concentration of application. If the concentration is higher than a certain level, the membrane may be damaged. In order to prevent the membrane from damaging, adding additional reducing agents to RO membrane unit is necessary. For example, adding reducing agents to the concentrated water of the first stage RO so as to neutralize the concentrated stabilized chlorine (JP 2010-201313), which increases the complexity of the operation and is hard to control. Thus, it is desired to find a proper method for inhibiting the growth of biofouling in membrane separation device, especially in reverse osmosis membrane separation device or nanofiltration membrane separation device.